The invention relates to devices enabling the identification of objects at a short distance from the latter, without direct contact with them, and comprising on the one hand an interrogator-reader member, also referred to as a "reader" below, arranged so as to generate an alternating voltage at high frequency F in an inductive impedance and to receive and identify binary response signals appearing in the form of coded sequences of electrical pulses modulating the frequency F and succeeding at a frequency of formation f less than F, sequences applied inductively on said impedance, and on the other hand an electronic identification circuit mounted on each object to be identified, said circuit, also referred to as below "badge" below, being adapted to be inductively coupled to the above impedance by simple mutual approach, to be supplied electrically by the single alternating voltage F due to the simple fact of this coupling and to form response signals of the type indicated above, specific to the badge concerned, applicable to the reader by said coupling.
Such devices have, for exaple, been described in the U.S. Pat. No. 3,299,424.
These devices lend themselves particularly well to the following application: the reader is associated with a security apparatus of which it is desired to control the actuation, an apparatus such as a money distributor or an access lock to a protected premises, and the object to be identified is a portable card allocated to a user used to carrying out said actuation.
In such a case, to perform such actuation, it suffices for the user to bring his card up to the reader, the necessary inductive coupling being obtained without the approach being performed until actual contact between the card and the reader: in particular it is unnecessary here to introduce the card into a slot specially provided in said reader.
Such a technique is advantageous in that it relates not only to the simplicity and rapidity of the actuation, but also the possibility of providing the reader externally with a continuous protective which is not perforated nor slotted and thus is particularly resistant to vandalism.
Devices of the above type at present known have a certain interest, but their performance is limited in serveral areas such as the sensitivity afforded thereby and in the inability with such devices to provide miniaturization of the badges associated with the objects to be identified.
These limitations arise from the techniques adopted with respect to the supply of power to each badge and with respect to the structure of binary coded response signals.
According to this technique, it is the voltage developed at the terminals of a resonant circuit comprising the inductive coupling impedance of the badge which is used both for the supply of this badge and for the inductive transmission of the coded response signals to the reader.
This voltage, which is formed by short circuiting by means of an electronic switch the above resonant circuit in a sequence or rate programmed according to the code to be identified, appears in the form of a train of coding pulses succeeding each other at the frequency f and modulating the frequency F.
Only the portions of large amplitude of this pulse train, corresponding to the periods outside short circuiting can be used for the supply, which places in operation the charging of a filtering capacitor, this capacitor thereafter discharging during the short circuits.
If therefore the duration of the short circuits is too great, there is the risk of the supply being disturbed.
Now this duration cannot be lowered below a predetermined limit due to the fact on one hand that the frequency F is limited upwards (particularly to a value of the order of 150 kHz), for reasons of congestion of Hertzian space and that on the other hand the modulation of each of the bits composing the binary signals must be fairly precise to permit precise detection of the beginnings and ends of these bits.
Several proposals have been made to attempt to overcome this drawback.
For example, it has been proposed in the patent U.S. Pat. No. 4,196,418 to divide the time into cycles of two periods each succeeding each other at a frequency g less than f, the first period of each cycle being used for the supply of power to the badge and the second period, for the formation and the transmission of coded response signals.
This technique also offers numerous drawbacks such as the considerable bulk of the supply capacitor necessary and the obligation of limiting the length of the coded signals if it is desired to avoid the necessity of dividing them up so as to distribute them over several successive coding periods.
It has also been proposed in U.S. Pat. No. 4,333,072 to short-circuit a resistance which is not in the resonating circuit of the badge, which limits all the more the discharge of the supply capacitor.
This solution has the drawback of considerably reducing the signal/noise ratio or more precisely the ratio between the respective amplitudes of the portions, of the coded binary signals corresponding respectively to the 0 bits and to the 1 bits, which reduces all the more the sensitivity of the identification.